1. Field of the Invention
The present invention relates to a belt unit and an image forming apparatus.
2. Description of the Related Art
A belt used in a belt unit of image forming apparatuses sometimes drifts. For controlling and preventing such belt drifting, various technologies are developed. For example, Japanese Patent No. 3402896 discloses a conventional technology for preventing a drifting of a conveyor belt in a thrust direction by using flanges. Specifically, the flanges having outer diameters larger than that of a driven roller around which the conveyor belt is extended are arranged on both ends of the driven roller such that the conveyor belt is sandwiched by the flanges. Therefore, the drifting of the conveyor belt in the thrust direction can be prevented by the flanges. Furthermore, Japanese Patent Application Laid-Open No. 2000-155480 discloses another conventional technology for preventing the belt drifting by arranging flanges with springs. Specifically, the flanges are arranged on both ends of a shaft of a driven roller and connected to side plates via springs. The springs apply spring pressure to the flanges in the thrust direction, and thereby the belt drifting can be prevented.
However, the flanges in the former conventional technology are formed in general circular shapes although they are formed to have the outer diameters larger than that of the driven roller. Therefore, if the conveyor belt drifts during its rotation and side edges of the conveyor belt hit the flanges, the flanges may be removed from the roller. The flange is generally made of resin by injection molding instead of metal for realizing cost reduction and weight reduction. Such a flange can be easily fitted into the roller; however, the flange often causes moment toward a direction away from the conveyor belt, causing the flange to be removed from the roller. Furthermore, with the latter conventional technology, it is necessary to have a space for arranging the springs in the thrust direction. Therefore, it is difficult to reduce a size of the belt unit. Moreover, necessary components increase because of the springs, increasing necessary costs.
Still another conventional technology is known as shown in FIG. 12, which is a cross sectional top view of flanges 23. Specifically, the flanges 23 are arranged on both ends of a roller 31 and each of the flanges 23 includes a pair of cylindrical portions 23a arranged opposite to each other across a rotation shaft 32 (hereinafter, “shaft 32”). An outer periphery of the cylindrical portion 23a is in contact with an inner periphery of the roller 31 thereby the cylindrical portion 23a supports the roller 31. Similarly, an inner periphery of the flange 23 is in contact with the shaft 32 thereby the flange 23 supports the shaft 32. Thus, the flanges 23 can prevent a drifting of a belt 8. However, although the flanges 23 have outer diameters larger than that of the roller 31, their thickness is relatively thin and uniform in a radial direction. Therefore, if the roller 31 is subjected to external force from the belt 8 extended around the roller 31, the flanges 23 may be removed from the roller 31 and thereby side edges of the belt 8 cannot be controlled at a predetermined position.
More specifically, the flanges 23 are provided by impaction (injection) of the cylindrical portions 23a into the roller 31. As shown in FIG. 13, the roller 31 with the flanges 23 is mounted on a belt unit in an image forming apparatus such that both ends of the shaft 32 of the roller 31 are mounted on side plates 15 via springs 16. The springs 16 apply spring pressure to the shaft 32 to apply tension to the belt 8. With this configuration, a line of action of force Fa does not match a line of action of force Fb. Therefore, moment toward a direction indicated by an arrow in FIG. 13 occurs on the flange 23 and the cylindrical portions 23a. The moment about a point of an outer edge Bout causes force toward a direction along which the flange 23 is removed from the roller 31. When the roller 31 rotates, the force is repeatedly applied as load onto the flange 23, resulting in causing the flange 23 to be removed from the roller 31 at an endurance time. For preventing such removal of the flange 23, it is possible to attach an E ring to the shaft 32 at a position farther from a center position of the shaft 32 than the flange 23 in the thrust direction. However, operations for attaching the E ring are necessary, increasing an operation procedure, and the E ring causes a problem such as friction of a sliding surface between the flange 23 and the E ring.